Centrifugal countercurrent contact systems



Jan 25, 1966 w. J. PODBIELNIAK 3,231,185

CENTRIFUGAL COUNTERCURRENT CONTACT SYSTEMS Filed Feb. '7, 1963 2 Sheets-Sheet 1 A P E M,M M

meg a Jan. 25, 1966 W. J. PODBIELNIAK 3,231,185

CENTRIFUGAL COUNTERGURRENT CONTACT SYSTEMS Filed Feb. 7, 1963 2 Sheets-Sheet 2 QZI 'g agkz/ J PM United States Patent 3,231,185 CENTRIFUGAL COUNTERCURRENT CONTACT SYSTEMS Walter J. Po'dbielniak, Chicago, Ill., assignor to Dresser Industries, Inc., a corporation of Delaware Filed Feb. 7, 1963, Ser. No. 257,026 Claims. (Cl. 233-15) This invention relates to improvements in centrifugal conntercurrent contact apparatus and in processes wherein two immiscible liquids of different densities are brought into countercurrent contact with each other in such apparatus.

Countercurrent contact apparatus having an interior working space and having partial barriers to radial flow within the working space are well known in the art.

Apparatus wherein the partial barriers are substantially cylindrical perforated walls and particularly concentric perforated rings or a perforated spiral are particularly well suited for countercurrent contact. A typical ap paratus utilizing concentric rings is described in my US. Patent No. 2,670,132, of February 23, 1954. A typical centrifugal conntercurrent contact apparatus using a perforated spiral to provide the barriers is described in my US. Patent No. 2,286,157, of June 9, 1942.

In centrifugal countercurrent contact apparatus wherein a working space rotates about a shaft it has been the practice to introduce and withdraw the separate liquid streams through paths at or near the axis or shaft. Where two inlet and two outlet streams are to be accommodated, it has been the practice to provide in the shaft an axial passageway extending toward the working space from each end of the shaft and an annular passageway concentric with each axial passageway and also extending inwardly from each end of the shaft.

Seals are required between the stationary and rotating elements of each of the four paths described to prevent leakage of liquid to the outside and to prevent leakage between adjacent paths. The principal problem has been the prevention of leakage between adjacent paths since such leakage can contaminate the product;

In accordance with one aspect of this invention the construction and maintenance of one of the seals in the system is simplified by substituting therefor a tortuous passage of high flow resistance.

In accordance with this aspect of the invention I have found that a tortuous passage of high flow resistance may be provided in place of the seal ordinarily used to keep separate the incoming heavy liquid and outgoing light liquid which pass each other in adjacent paths at one end of the shaft of the apparatus. When the pressures in these adjacent streams are controlled so that the pressure of the outgoing light liquid is somewhat higher than the pressure of the incoming heavy liquid a small amount of light liquid is recycled back to the working space through the tortuous passage and the heavy liquid inlet. This small amount of light liquid recycle is not detrimental with respect to the centrifugal countercurrent contact process per se and is, in fact, beneficial since it provides an additional stage of contact for the portion of light liquid which is recycled.

In accordance with another aspect of this invention a portion of the heavy liquid outlet stream is recycled1 to the working space through the light liquid inlet in order to reduce the pressure requiremnet at the light liquid in let, as will be explained more fully herein below. 1A5 an added benefit, the recycle of heavy liquidalso provides an added contacting stage for the portion of heavy liquid thus recycled.

For convenience, this invention will be described with,

FIG. 3 is a detail cross section of the other end of the inner shaft element; and

FIG. 4 is an enlarged detail cross section showing a portion of the tortuous passage provided in one embodiment of this invention.

Referring to the drawings, rotor 11 includes rotatable shaft 12, mounted and driven to rotate at a high rate of speed, for example, from about 1,000 to 5,000 rpm. or even higher. The shaft is maintained in a horizontal direction and serves as axis to the remainder of the rotor structure. The supporting means and mounting means are not shown.

The rotor is formed, as a cylindrical casing made up of two side plates 13 and 14 and a peripheral cylindrical member 16 rigidly secured to each other and to the shaft to form a closed chamber.

Within the closed chamber, annular passageways of progressively increasing radius are provided. The passageways are preferably formed by concentric rings or bands 17 which are perforated to permit limited flow of liquid in a radial direction but which provide substantial barriers to such flow. The distance between the bands may vary considerably from about 0.15 inch to about 2 inches depending on the physical properties of the liquids to be contacted. If desired the distance between bands may be varied, for example, by making the distance between bands a function of the radial distance of the axis of rotation, as shown in my US; Patent No. 2,670,132, referred to above.

The shaft 12 extends outwardly beyond side plates 13 and 14 and at each end comprises an inner cylindrical member 12a defining an axial passageway and an outer cylindrical member 121) defining an annular passageway between itself and the inner cylindrical member 12a.

' The light liquid is introduced through axial passageway 18 which leads into the shaft from the left, as seen in FIG. 1. Conduit 19 leads from the axial passageway into the working space of the rotor to a location near the periphery thereof.

Light liquid is passed into the axial passageway 18 Patented Jan. 25, 1966 through stationary pipe 21 which is fixed in stationary cap 22 and which is held in juxtaposition to the open end of the rotatable shaft to permit continuous liquid flow into passageway 18. The juxtaposed facings of inner shaft member 12a and inlet pipe 21 comprise a seal 23 composed of a hard alloy member 23a on the shaft and a soft carbon member 23b on the stationary pipe.

Heavy liquid is introduced into the system through pipe 26 leading to axial passageway 24 at the right end of shaft 12. Stationary cap 27 is similar to cap 22 at the opposite end of the shaft. Conduit 28 connects with axial passageway 24 to lead the heavy liquid into the working space of the rotor to a location near the axis thereof.

During the operation of the countercurrent contact process the light liquid passes inwardly through the perforated rings from passageway to passageway while the heavy liquid passes outwardly producing excellent countercurrent contact between the two liquids. Heavy liquid which has worked its way outwardly to the outermost passage of the rotor passes through conduit 29 to annular passageway 31 on the left side of the shaft, as shown in FIG. 1.

The juxtaposed facings of outer shaft member 12a and a portion of cap 22 comprise a seal 23 similar to seal 23 described above and composed of a hard alloy member 32a on the shaft and a soft carbon member 32b on the stationary cap.

The seal 23, described above, is a seal between light liquid in the axial passageway and heavy liquid in the annular passageway. The seal 32 is a seal between heavy liquid in the axial passageway and the atmosphere.

' The annular passageway 31 continues into cap 22 and communicates with the open end of stationary pipe 33 to permit heavy liquid to be withdrawn from the systern.

Light liquid which has passed toward the axis of the Workings space is withdrawn through conduit 34 and annular passageway 36 which leads into cap 27. There is a seal 38 similar to seal 32 described above to prevent leakage of light liquid to the atmosphere. A stationary .pipe 39 leads into cap 27 to permit the withdrawal of light liquid from the system.

At the juxtaposition of the ends of stationary heavy liquid inlet pipe 26 and the right end of inner shaft member 12a there is no seal similar to seal 23 on the opposite end. Instead, as shown most clearly in FIGS. 2 and 4, there is a labyrinthine passageway 41 provided by juxtaposed plates 41a and 41b. Plate 41a is mounted on the end of inner shaft member 12a and contains a series of circular ridges 42. Plate 41b is mounted on the end of the stationary pipe 26 and contains a series of circular grooves or channels 43 into which the circular ridges of plate 41a may be fitted. There is close clearance (of the order of about 0.001) between plates 41a and 41b so that liquid may pass between the plates, although the resistance to'flow is high.

As will be described more fully hereinbelow, the pressures on the light liquid outlet stream and the heavy liquid inlet stream are controlled so that the former is slightly higher than the latter. A pressure diiferential between about 0.5 and about p.s.i. between these streams is suitable. Under such conditions there is a slight flow of liquid inwardly between plates 41a and 41b. The liquid flow lubricates the surfaces of plates 41a and 41b and permits substantially frictionless rotation of the former relative to the latter.

The amount of flow through the tortuous passage of labyrinthine passageway 41 may be controlled by varying the clearance between plates 41a and 41b and/or by varying the pressure differential between the streams. The amount of flow may be barely measurable if it is desired that the tortuous passageway approximate a seal in operation, or it may be a more substantial portion of the total light liquid outflow if it is desired to recycle a portion of light liquid for an additional stage of contact with the heavy liquid. In the latter case a flow between the plates of the order of about 5 to 50 percent of the total light liquid may be maintained.

The seals 23, 32 and 38 have been described above in a relatively simple embodiment. In an operating centrifugal contact apparatus, such seals would remain effective for only a relatively short time since this simple embodiment provides no means for taking up wear. In a machine intended to operate for substantial periods without adjustment a more complex seal such as that shown in FIGURE 3 must be used.

In FIGURE 3, elements 12a, 21, 23a and 23b are identical to the similarly numbered elements of FIG- URE 1. In the embodiment of FIGURE 3, however, conduit 21 does not abut directly onto the end of cylindrical member 12a. Instead, it communicates with a movable end member 51 which is held in abutment with the end of member 12a by the compression of spring 52 against shoulder 53 of the end member;

The inside diameter of pipe 21 is widened at its end to receive the movable end member 51 and a movable sealing ring 54 permits the end member to move in an axial direction relative to pipe 21 without liquid leakage and thereby to compensate for wear on the sealing element 23b which is mounted on the movable end member 51.

Similar mechanisms are used to compensate for wear on seals 32 and 38 but not on the labyrinthine passageway 41. Plates 41a and 41b of the labyrinthine passageway are out of contact with each other and not subject to wear.

Theelimination of the complex seal structure between the light liquid outlet and heavy liquid inlet and its replacement by the relatively simple labyrinthine passageway is one of the principal advantages of this feature of the invention.

At the opposite end of the shaft from the labyrinthine passageway a portion of the outgoing heavy liquid passing through stationary pipe 33 may be drawn off through bypass 44 controlled by valve 46 and recycled into the working space through light liquid inlet pipe 21. A pump 47 is provided in this bypass line since the pressure in the heavy liquid outlet line is normally substantially lower than the pressure in the light liquid inlet line.

Recycle of a portion of the heavy liquid outlet stream permits an additional stage of contact of the recycled portion with light liquid and also provides a substantial advantage in permitting lower pressures to be used on the light liquid inlet stream.

In order to provide the desired liquid flow through the contact apparatus there are pressure diiferentials which must be overcome. One of the pressure differentials is the differential between the pressure at the light liquid inlet and the pressure at the heavy liquid outlet.

The conduit between the light liquid inlet and the periphery of the interior working space and the conduit between the heavy liquid outlet and the periphery of the interior working space may be considered as the legs of a manometer. Since conduits 27 and 28 both extend toward the periphery of the rotor, these legs are of about equal length. Since the leg leading from the light liquid inlet ordinarily contains only light liquid, it exerts less pressure than the leg leading to the heavy liquid outlet since the latter leg contains liquid having a higher density. In order to force light liquid into the system, there must be sufiicient pressure behind the light liquid stream to overcome the difference in pressure between the two manometer legs. In the usual centrifugal countercurrent contact apparatus the heavy liquid outlet is at about atmospheric pressure while the light liquid inlet is maintained at about p.s.i.g. to overcome the pressure differential.

By recycling a portion of the heavy liquid product stream through axial passageway 24 and radial conduit 27, the average density in conduit 27 is increased to a value above the density of pure light liquid and therefore closer to the density of the heavy liquid in conduit 28.

This tendsto lessen the pressure differential between the two manometer legs and therefore lessen the pressure requirement on the light liquid feed stream.

The higher the proportion of heavy liquid recycled to the working space through the light liquid inlet the greater will be the increase in average liquid density in conduit 27 and the less will be the pressure requirement on the light liquid line. For example, the recycle of an amount of heavy liquid which is about 25 percent by volume of the amount of light liquid feed will reduce the pressure required at the light liquid inlet by about percent. Similarly the recycle of an amount of heavy liquid approximately equal in volume to the amount of light liquid feed will reduce the pressure requirement by about 50 percent. V Just as the pressures in the light liquid inlet line and the heavy liquid outlet line may be calculated by a consideration of the manometer legs in conduit lines 27 and 28, the relationship between the pressures in the heavy liquid inlet line and the light liquid outlet line may also be determined by studying the relationship of the respective manometer legs associated with each stream. Although conduit 19 has been described above as leading to a position within the working space close to the axis, it is to be noted, as shown in FIGURE 1, that it actually extends to a position somewhat displaced from the axis and terminates in a space between two concentric rings near the shaft. The heavy liquid in conduit 19 may be considered as one leg of a manometer while the other leg may be considered to comprise the light liquid in conduit 34 and a column of liquid through the working space from the shaft to the space between the rings in which conduit 19 terminates. The density of liquid in conduit 19 is that of the heavy liquid while the density of liquid in the other leg is the density of the light liquid in conduit 34 and the average liquid density in the column of liquid extending tofthe space in which conduit 19 terminates. The latter average density may approach the density of the light liquid or may approach the density of the heavy liquid depending on the relative thickness of the light and heavy liquid layers between the inner rings.

In any case the density of liquid in conduit 19 is higher than the effective density of liquid in the other leg of the manometer. The conduit 19 is also somewhat longer than the other leg of the manometer since it extends to axial passage 18 while the other leg extends only to angular passage 36.. Both the higher density and longer length of themanometer leg in conduit 19 assure a lower pressure in the heavy liquid inlet line 18 than in the light liquid outlet line 36. The lower pressure in axial passageway 18 assures flow of liquid only inwardly through the labyrinthine passageway 41. While the relation of pressures in, conduits 18 and 36 to each other is determined solely by consideration of the pressures developed in the respective manometer legs discussed above, the relationship of the pressures in conduits 18 and 36 to the light liquid inlet pressure, which is the highest pressure in the system, is determined by the position of the principal interface in the system,"as described hereinbelow.

In centrifugal countercurrent contact apparatus of the type described herein there is a series of interfaces between light liquid and heavy liquid, one interface in each space between the concentric rings. All of the interfaces, except one, are maintained in a state of-unstable pressure equilibrium and are termed. auxiliary interfaces. In each auxiliary interface, one of the layers of liquid (the layer of the liquid flowing toward the pressure equilibrium position) is a very thin layer.

'One interface, called the principal interface is in a state of stablepressure equilibrium and has liquid layers of substantial thickness on both sides thereof. The principal interface is in a position which may 'be varied, as desired, by controlling the pressures in the conduit lines, and particularly the pressure in the light liquid outlet,

A relatively high pressure in the light liquid outlet, approaching the pressure at the light liquid inlet will assure an average density in the extractor approaching that of the light liquid in the light liquid incoming conduit.

The pressure developed in the light liquidincoming conduit must also balance the pressure developed in the working space of the rotor in order for incoming flow to take place. This may be considered as a third manometer with the light liquid in the inlet conduit as one leg and a column of liquid in the working space as the other.

When the pressure in the light liquid outlet necessitates an average liquid density in the working space approaching that of the light liquid, the principal interface is in a position close to the periphery and the heavy liquid layers at the auxiliary interfaces are thin.

Conversely, when the pressure in the light liquid outlet is relatively low, approaching that of the heavy liquid outlet, then the average density of the liquid in the working space must be close to the density of heavy liquid in the heavy liquid outgoing conduit. In such a case the principal interface is close to the axis and the layers of light liquid at the auxiliary interfaces are thin.

In the third manometer, the average density of the column of liquid within the rotor working; space is determined by the position of the principal interface. When the principal interface is maintained at an inner position, close to the axis, most of the working space is filled with heavy liquid and the manometer leg within the working space has a higher average density than the manometer leg of the light liquid inlet conduit.

The position of the principal interface is thus controlled by controlling the back pressure on the light liquid outlet line, as by valve 61. When a high back pressure is maintained, approaching the pressure in the light liquid inlet line, the principal interface is close to the axis. A low pressure on the light liquid outlet line moves the principal interface to a position close to the periphery.

Adjustment of valve 61 determines the pressure in the light liquid outlet line directly and the pressure in the heavy liquid inlet line indirectly since the latter is maintained somewhat lower than the former by the relationship of the, lengths of conduits 28 and 34.

While the invention has been described with respect to particularly illustrated embodiments it is to be understood that other embodiments may be employed if desired. In place of the particular form of labyrinthine passageway described above, other forms may be used.

In addition, while the invention has been described with particular reference to centrifugal countercurrent contact apparatus in which concentric cylindrical barriers are maintained within the working space, it is to be understood that other types of partial barriers such as a perforated spiral or packing or gauze may be used without departing from the essence of this invention.

The above detailed description of this invention has been given for clearness of understanding only. No unnecessary limitations should be understood therefrom, as modifications will be obvious to those skilled in the art.

I claim: i i

1. In a centrifugal countercurrent contact apparatus for effecting intimate contact between at least partially immiscible liquids of ditferent densities having a rotor with an interior working space, said working space having a plurality of partial barriers to radial flow within the rotor, inlet means for supplying lighter liquid to the interior of the rotor in the proximity of its periphery, inlet means for supplying 'heavierliquid to the rotor in the proximity of its axis, and separate outlet means for discharging heavier liquid and lighter liquid from said rotor, each of said inlet means and said outlet means including stationary and rotatable elements; the improvement which comprises a conduit exterior of said working space be-,

tween the outlet means of one of said liquids and the inlet means of the other, said aliquot portion being identical in composition to the remainder of said discharge stream to permit the passage of an aliquot portion'of a discharge stream of one of said liquids to an inlet stream of the other liquid.

2. In a centrifugal countercurrent contact apparatus for effecting intimate contact between at least partially immiscible liquids of different densities having a rotor with an interior working space, said working space hav ing a plurality of partial barriers to radial flow within the rotor, inlet means for supplying lighter liquid to the interior of the rotor in the proximity of its periphery, inlet means for supplying heavier liquid to the rotor in the proximity of its axis, and separate outlet means for discharging heavier liquid and lighter liquid from said rotor, each of said inlet means and said outlet means including stationary and rotatable elements; the improvement which comprises a conduit exterior of said working space between said heavier liquid outlet means and said lighter liquid inlet means, said aliquot portion being identical in composition to the remainder of said discharge heavier liquid to permit the passage of an aliquot portion of a discharge heavier liquid into an inlet stream of lighter liquid.

3. In a centrifugal countercurrent contact apparatus for effecting intimate contact between at least partially immiscible liquids of different densities having a rotor with an interior working space, said working space having a pluralty of partial barriers to radial flow within'the rotor, inlet means for supplying lighter liquid to the interior of the rotor in the proximity of its periphery, inlet means for supplying heavier liquid to the rotor in the proximity of its axis, and separate outlet means for discharging heavier liquid and lighter liquid from said rotor, each of said inlet means and said outlet means including stationary and rotatable elements; the improvement which comprises a conduit exterior of said working space between said lighter liquid outlet means and said heavier liquid inlet means to permit the passage of an aliquot portion of discharge lighter liquid to an inlet stream of heavier liquid, said aliquot portion being identical in composition to the remainder of said discharge stream.

42 In a centrifugal countercurrent contact apparatus for effecting intimate contact between at least partially immiscible liquids of different densities having a rotor with an interior working space, said working space having a plurality of partial barriers to radial flow within the rotor, inlet means for supplying lighter liquid to the interior of the rotor in the proximity of its periphery, inlet means for supplying heavier liquid to the rotor in the proximity of its axis, and separate outlet means for discharging heavier liquid and lighter liquid from said rotor, each of said inlet means and said outlet means including stationary and rotatable elements; the improvement which comprises a labyrinthine conduit of high resistance to flow exterior of said working space between said lighter liquid outlet means and said heavier liquid inlet means.

5. The centrifugal contact apparatus of claim 1 wherein said partial barriers are concentric perforated rings disposed about the axis of said rotor as a center.

6. The centrifugal contact apparatus of claim 1 wherein said partial barriers comprise a perforated spiral generated from the axis of said rotor.

7. The centrifugal contact apparatus of claim 1 wherein said partial barriers comprise discrete packing elements in contact with each other.

8. The centrifugal contact apparatus of claim 2 wherein said conduit contains a pump.

9. The centrifugal contact apparatus of claim 3 wherein said heavier liquid inlet means includes a conduit axial to said rotor and said lighter liquid outlet means includes a conduit annular to said axial conduit.

10. The centrifugal countercurrent contact apparatus of claim 4 wherein said labyrinthine conduit is bounded by two closely spaced plates facing each other, one of said plates having circular ridges on its surface facing the'op- 8. posite plate, and the other plate having circular grooves into'which said circular ridges may be fitted.

11. In a method for effecting countercurren't contact between at least partially immiscible liquids" of different densities wherein a feed stream of light liquid is introduced into the interior and near the periphery of acentrifugal contacting zone which rotates'about an axis and which contains barriers to radial flow, a feed stream of heavy liquid is introduced into the interior and near the axis of said centrifugal contacting zone, a discharge stream of heavy liquid is withdrawn'froni near the periphery of said centrifugal contacting zone and a' discharge stream of light liquid is withdrawn from near the axis of said centrifugal contacting zone, the improvementjwhich com prises feeding an aliquot portion of a discharge stream from said centrifugal contact zone into the feed stream of the liquid of different density therefrom being introduced to the interior of said centrifugal contacting zone, said aliquot portion being identical in'composition to the remainder of said discharge stream.

12. In a method for effecting countercurrent contact between at least partially immiscible liquids of different densities wherein a feed stream of light liquid is introduced into the interior and near the periphery of a centrifugal contacting zone which rotates about an axis and which contains barriers to radial fiow, a feed stream of heavy liquid is introduced into the interior and near the axis of said centrifugal contacting zone, adischarge stream of heavy liquid is withdrawn from near the periphery of said centrifugal contacting zone and a discharge stream of light liquid is withdrawn from near the axis of said centrifugal contacting zone, the improvement which comprises feeding an aliquot portion 'of' said light liquid discharge stream into said feed stream of heavy liquid being introduced into said contactingzone, said aliquot portion being identical in composition to the remainder of said discharge stream. I

13. In a method for effecting countercurrent contact between at least partially immiscible liquids of different densities wherein a feed stream of light liquid is introduced into the interior and near the periphery of a centrifugal contacting zone which'rotates about an axis and which contains barriers to radial flow, a feed stream of heavy liquid is introduced into the interior and near the axis of said centrifugal contacting zone, a discharge stream of heavy liquid is withdrawn from near the periphery of said centrifugal contacting zone and a discharge'stream of light liquid is withdrawn from near the axis of said centrifugal contacting zone, the'improvement which comprises feeding an aliquot portion of said heavy liquid discharge stream into said feed stream of light liquid being introduced into said contacting zone, said aliquot portion being identical in composition to the remainder of said discharge stream.

14. In a method for effecting countercurrent contact between at least partially immiscible liquids of different densities wherein a feed stream of light liquid is introduced into the interior and near the periphery of a centrifugal contacting zone which rotates about an axis and which contains barriers to radial flow, a feed stream of heavy liquid is introduced into the interior and near the" axis of said centrifugal contacting zone, a discharge stream of heavy liquid is withdrawn from near the periphery of said centrifugal contacting zone and a discharge stream of light liquid is withdrawn from near the axis of said centrifugal contacting zone, the improvement which comprises passing an aliquot portion. of said light liquid discharge stream through a tortuous passage of high flow resistance to feed said portion into said feed stream of heavy liquid being introduced into said contacting zone, said aliquot portion being identical in composition to the remainder of said discharge stream.

15. The method of claim 14 wherein said light liquid discharge st eam ismaintainedata" pressure higher than about 0.5 and 5 p.s.i.

9 the pressure of said heavy liquid feed stream by between References Cited by the Examiner UNITED STATES PATENTS 5 6/1914 Hall 233-19 7/ 1917 Sharples 233-14 8/1933 Peltzer et al. 233-14 1/ 1939 Ruda. 10 4/ 1939 Podbielniak 261-83 X 9/ 1939 Podbielniak. 7/ 1940 Podbielniak. 6/1942 Podbielniak 233-15 X 1/1953 Abbott 233-14 15 Staaif 233-14 Brewer 233-15 Podbielniak 233-15 Pomeroy 233-14 X Podbielniak 233-15 Sullivan.

Thurman 233-31 X Jacobson 233-14 Podbielniak 23 3-15 Mitchell.

M. CARY NELSON, Primary Examiner.

ROBERT F. BURNETT, Examiner.

H. KLINKSIEK, Assistant Examiner. 

1. IN A CENTRIFUGAL COUNTERCURRENT CONTACT APPARATUS FOR EFFECTING INTIMATE CONTACT BETWEEN AT LEAST PARTIALLY IMMISCIBLE LIQUIDS OF DIFFERENT DENSITIES HAVING A ROTOR WITH AN INTERIOR WORKING SPACE, SAID WORKING SPACE HAVING A PLURALITY OF PARTIAL BARRIERS TO RADIAL FLOW WITHIN THE ROTOR, INLET MEANS FOR SUPPLYING LIGHTER LIQUID TO THE INTERIOR OF THE ROTOR IN THE PROXIMITY OF ITS PERIPHERY, INLET MEANS FOR SUPPLYING HEAVIER LIQUID TO THE ROTOR IN THE PROXIMITY OF ITS AXIS, AND SEPARATE OUTLET MEANS FOR DISCHARGING HEAVIER LIQUID AND LIGHTER LIQUID FROM SAID ROTOR, EACH OF SAID INLET MEANS AND SAID OUTLET MEANS INCLUDING STATIONARY AND ROTATABLE ELEMENTS; THE IMPROVEMENT WHICH COMPRISES A CONDUIT EXTERIOR OF SAID WORKING SPACE BETWEEN THE OUTLET MEANS OF ONE OF SAID LIQUIDS AND THE INLET MEANS OF THE OTHER, SAID ALIQUOT PORTION BEING IDENTICAL IN COMPOSITION TO THE REMAINDER OF SAID DISCHARGE STREAM TO PERMIT THE PASSAGE OF AN ALIQUOT PORTION OF A DISCHARGE STREAM OF ONE OF SAID LIQUIDS TO AN INLET STREAM OF THE OTHER LIQUID. 